Fuel cells have recently been developed as an alternative power source, such as for electrical vehicles. A fuel cell is a demand-type power system in which the fuel cell operates in response to the load imposed across the fuel cell. Typically, a liquid hydrogen containing fuel (e.g., gasoline, methanol, diesel, naptha, etc.) serves as the fuel supply for the fuel cell once it is converted to a gaseous stream containing hydrogen. This is accomplished when the hydrogen-containing fuel is passed through a fuel reformer to convert the liquid fuel to a hydrogen gas (10-75% depending on the liquid fuel) that usually contains other passivating gas species such as carbon monoxide, carbon dioxide, methane, water vapor, oxygen, nitrogen, unburned fuel and, in some cases, hydrogen sulfide. The hydrogen is then used by the fuel cell as a fuel.
In many prior art systems, the fuel reformer for reforming a hydrocarbon based fuel into a gaseous reformate stream includes a hydrogen membrane purification unit that consists of a highly perm-selective membrane that preferentially passes hydrogen molecules. In operation, high pressure reformate is flowed past one side of the membrane and lower pressure, normally pure hydrogen, is collected on the other side (permeate) of the membrane. The difference in hydrogen partial pressures across the membrane supplies the driving force for hydrogen separation. The permeate hydrogen pressure should never exceed the reformate hydrogen partial pressure. Further, the higher the permeate pressure, the larger the amount of reformate hydrogen that will exit the membrane separator without passing across to the permeate side.
The pure gaseous hydrogen is, typically conveyed to a metal hydride system that stores gaseous hydrogen and functions as both a hydrogen load leveling device and as a reservoir of hydrogen for use during vehicle start-up (while the hydrocarbon reformer is warming to operating temperature). The metal hydride system serves as a load leveling device by absorbing (storing) hydrogen gas from the reformer/H.sub.2 membrane system when reformer output exceeds fuel cell hydrogen consumption and desorbing (delivering) stored hydrogen when reformer output is less than fuel cell consumption. Hydrogen is absorbed into a metal hydride alloy when gas pressure exceeds the hydride equilibrium pressure (for a given temperature) and hydrogen is desorbed from the metal hydride when gas pressure is below the hydride equilibrium pressure. In general, high hydrogen pressure is desired to charge a metal hydride bed and low hydrogen pressure is desired to discharge the bed.
One problem with current reformer fed fuel cell systems is that they are relatively large, heavy and expensive. This is due in part to the existence of contaminants and dilutents (i.e., non-hydrogen gases) in the hydrogen fuel provided to the fuel cell. These contaminants and dilutents cause a relatively significant reduction in the power production per unit weight and volume of the fuel cell.
A further problem with current fuel cell systems is that they typically require a hydrogen compressor in order to increase the hydrogen pressure to the required metal hydride absorption pressure. The inclusion of a hydrogen compressor also adds increased cost and weight to the system.
Additionally, current onboard reformed fuel cell systems have a relatively slow reaction time to start up electric vehicles than it takes for conventional vehicle systems.